Summary The antitumor efficacy of type I interferon (IFN-/) therapy is variable since it can induce either tumor cell growth inhibition or apoptosis. The underlying molecular mechanisms of type I IFN-induced apoptosis remain largely unknown. Defining the signaling pathways activated by type I IFNs leading to tumor cell destruction are of clinical importance. Our preliminary data indicates that a deficiency in STAT2 prevents cells from undergoing IFN- -induced apoptosis and this defect correlates with impaired expression of interferon stimulated genes (ISGs). We have found that the pro-apoptotic protein Bim, which disrupts mitochondrial integrity, is activated by type I IFNs in a STAT2-dependent manner. Importantly, crosstalk between Bim and STAT2 signals likely exists since the apoptotic activity of type I IFNs is impaired in mouse embryonic fibroblasts deficient in either Bim or STAT2. Based on our data, our hypothesis is that type I IFN-induced STAT2 activity regulates the activation of the mitochondrial dependent death pathway by modulating the pro-apoptotic activities of BH3 domain only Bcl-2 proteins. Specific Aims: 1) Determine the mechanism by which STAT2 modulates Bim activation. 2) Characterize conserved residues in STAT2 and determine whether they modulate type I IFN signaling and Bim activation. 3) Determine how STAT2 is required for the in vivo antitumor effects of type I IFNs. Significance: These results will provide insights into the signaling mechanisms and antitumor efficacy of type I IFN-induced apoptosis that are regulated by STAT2 and Bim.